Because of the high biological activity of such 1-hydroxylated vitamin D compounds and their potential utility for the treatment of many diseases related to calcium metabolism disorders there has been much interest in chemical processes for their preparation. Almost all of the reported syntheses involve the 1.alpha.-hydroxylation of suitable steroids (such as cholesterol) which are subsequently converted to the desired 1.alpha.-hydroxyvitamin D compounds (see Schnoes and DeLuca, in Bioorganic Chemistry, vol. 2, Chapter 12, pp. 299-335, edited by E. E. van Tamalen, Academic Press, Inc., New York, 1978).
A novel alternative process introduced recently (Paaren et al, Proc. Nat. Acad. Sci. USA 75, 2080, 1978) entails the preparation of 1.alpha.-hydroxyvitamin D compounds by acid catalyzed solvolysis of 1.alpha.-0-acyl-3,5-cyclovitamin D compounds which in turn are prepared from 3,5-cyclovitamin D compounds by allylic oxidation and acylation. The process may be illustrated by the following reaction, where R may be any steroid side chain and X is hydrogen or acyl, depending on the solvolysis condition chosen. ##STR1## This process however, as shown in the above scheme, leads to both 5,6-cis and 5,6-trans 1.alpha.-hydroxyvitamin D compounds (in a ratio of ca. 2:1 to 5:1 depending on conditions) from which the desired 5,6-cis-isomer must be separated by chromatography. Hence, the formation of 5,6-trans compound is a disadvantageous feature of this process (unless 5,6-trans material is desired), which reduces the yield of the desired 1.alpha.-hydroxy-5,6-cis-vitamin D compounds.